1. Related Field
The present invention relates to a steel-made Yankee cylinder having a cylindrical shell and end walls welded to the axial ends of the cylindrical shell.
2. Description of Related Art
In a paper making machine for making tissue paper, a newly formed fibrous web which is still wet is dried on a Yankee drying cylinder. The Yankee drying cylinder is typically filled with hot steam which may have a temperature of up to 180° C. or even more. The hot steam heats the Yankee drying cylinder such that the external surface of the Yankee cylinder reaches a temperature suitable for effective evaporation of water in a wet fibrous web such as a tissue paper web. The steam is normally pressurized to such an extent that the Yankee cylinder is subjected to substantial mechanical stress due to the internal pressure. The overpressure inside the Yankee cylinder during operation may be about 1 MPa (10 bar).
The weight of the Yankee cylinder as well as centrifugal forces may also contribute to the mechanical stress. The Yankee cylinder must be made to withstand such mechanical stress. Yankee drying cylinders have usually been made of cast iron but it is known that a Yankee cylinder can also be made of welded steel. EP 2126203 discloses a Yankee cylinder for drying paper which is made of steel and has a cylindrical shell joined to two ends through a respective circumferential weld bead made between opposing surfaces of each end and the cylindrical shell. The cylindrical shell is made such that, close to each of its end edges, it has a portion of cylindrical wall of a thickness gradually increasing from a zone of minimum thickness to a zone of maximum thickness in correspondence of which the circumferential weld bead is formed.
In addition to being strong enough to withstand mechanical stress, a Yankee drying cylinder should preferably also be easy to manufacture. Therefore, it is an object of the present invention to provide a design of a Yankee drying cylinder that allows the Yankee drying cylinder to be manufactured.